During the Cold War as well as in the wake of the 9/11 terrorist attacks, low yield tactical nuclear weapons have been deployed or, in the latter case, at least envisaged (again). Of particular interest in the early years of the 21st century have been so called bunker busters, nuclear weapons which can penetrate the ground and which can be used to destroy underground facilities, such as Iranian nuclear facilities. In this article, the role of tactical nuclear weapons during the Cold War and today is explained and the legality of the use of tactical nuclear weapons, in particular bunker busters, is examined from the perspective of international law, both in the context of the non-proliferation regime and, more importantly, the ius in bello, International Humanitarian Law.